Vapor-cell atomic clocks employ an RF-discharge lamp to generate the atomic clock signal. As a consequence, the performance of the atomic clock depends on the spectral output of the RF-discharge lamp, which in turn is determined by the detailed properties of the light-generating plasma within the lamp. The light emission characteristics of discharge lamps can change slowly over time, and this can affect the accuracy of the atomic clock.
All clocks measure time intervals by determining the elapsed phase of some stable oscillation. Every precision clock requires a precision frequency standard. Consequently, variations in a reference oscillator frequency, Δωclk(t), will give rise to time-interval errors. The oscillator frequency provides a tick-rate for the clock, and errors in the tick-rate imply that the clock is running too fast or too slow. In a crystal clock, the oscillation frequency is defined by the output of a free-running quartz crystal oscillator. As is well known, the free-running oscillations may be perturbed by oscillator temperature variations, pressure variations, and radiation. In the case of an atomic clock, the output frequency of the crystal is locked to an atomic resonance so that the determination of time-intervals takes on the stability of an atomic energy-level structure. As a consequence, atomic clocks are much less sensitive to the effects of temperature, pressure, and radiation.
A magnetic dipole interaction exists in Rubidium between the single orbiting valence electron and the atomic nucleus of the Rubidium atom. This subatomic magnetic interaction, termed the hyperfine interaction, causes the electronic and nuclear magnetic moments to align either parallel or antiparallel with one another. In order to employ this interaction for precise timekeeping, the output frequency of a quartz crystal oscillator at about 10.0 MHz is first multiplied up into the microwave regime and then modulated at some low frequency. The microwave signal at 6834.7 MHz then interacts with a vapor of Rb87 atoms, probing the hyperfine interaction by causing the atoms to switch, back and forth, between two hyperfine states, that is, the electronic and nuclear magnetic moments are first parallel, then antiparallel, now parallel again, and so on. The probing process can detect very small microwave frequency excursions from the center frequency of 6834.7 MHz because the Q of the response of the Rubidium atoms to the probing process is very high, on the order of 107. Employing phase-sensitive-detection, a feedback correction signal is derived from the probing process and is used to lock the crystal oscillator output frequency to the hyperfine interaction of the Rb87 atoms.
A Rubidium atomic clock system using a generic vapor-phase atomic clock design includes a Rb RF-discharge lamp that is excited by a 102 MHz signal vrf, a filter cell containing Rb85 vapor, a resonance cell containing Rb87 vapor, and a photodetector. The Rb lamp emits spectral lines in the near-IR, at 780 nm and 795 nm, also known as the lamp emission. After passing through the Rb85 vapor in the filter cell, the spectrum of the lamp emission is altered slightly so that the light can efficiently generate an atomic clock signal. The filtered lamplight prepares the atoms in the Rb87 resonance cell for interaction with microwaves in a process known as optical pumping, and additionally monitors the Rb87 atomic interaction with the microwaves. The Rb87 atomic response to the microwaves is the essential atomic clock signal. When the microwaves are tuned to the appropriate frequency, so that the Rb87 atoms strongly absorb the microwaves, the intensity of lamplight transmitted by the resonance cell decreases. When the microwave frequency is not tuned appropriately, the Rb87 atoms do not absorb the microwaves and the intensity of the transmitted lamplight remains unaffected. As such, the microwaves must be within about one part in 107 of the resonance frequency of the Rb87 Rubidium atoms in order to affect the transmission of the lamplight through the resonance cell.
In addition to producing the atomic clock signal, the lamplight disadvantageously slightly perturbs the atoms, altering the atoms natural microwave absorption resonance frequency and thereby the atomic clock frequency ωclk. This phenomenon is known as the light shift effect. The light shift effect depends on the intensity and spectrum of the lamplight. The light shift effect is an important effect in determining atomic clock performance. In particular, recent GPS on-orbit clock data clearly show that the lamp intensity can experience relatively sudden changes, which in turn disadvantageously give rise to sudden changes in the frequency of the clock. As a consequence, stabilization of the lamp emission results in stabilization of the atomic clock frequency, which in turn results in stabilization of the tick-rate of the clock and hence the ability of the clock to keep accurate time.
The RF-discharge lamp generates light via a weakly ionized alkali and noble-gas plasma. The plasma can generate acoustic ion waves, which are essentially bulk motions of the positive ions in the plasma. Under normal lamp operating conditions, where the Debye length is very small, at about 10−3 cm, the frequency of these acoustic ion waves, faco, follows a relatively simple dispersion law defined by a dispersion equation faco≅√(KTe/Mionλ2). In the dispersion law, Te is the effective plasma electron temperature, Mion is the ion mass, and λ is the wavelength of the plasma oscillation. With Te equal 2×103°K, Mion equal to 100 gms/mole, and λ equal to 2 L, where L is equal 1.5 cm and is the length of the lamp, the frequency faco of the acoustic ion waves is 14 kHz. The frequency faco of the acoustic ion waves depends on the electron temperature. The electron temperature will change over time as more or less RF power is coupled into the plasma. As such, the frequency faco of the acoustic ion waves will vary with time as the plasma temperature and power changes over time. The plasma temperature and power changes also affect the lamplight, leading to poor atomic clock performance via the light shift effect. The plasma temperature and power changes of the RF-discharge lamp have not been characterized nor stabilized in an atomic clock system leading to inaccurate atomic clock performance. These and other disadvantages are solved or reduced using the invention.